Field of the Invention
The present invention relates to a sensor usable for detecting an acceleration, a pressure, or more generally any physical magnitude for which a variation can cause a movable body to move relative to a stand. The invention is more particularly suitable for use in an application to an inertial acceleration sensor, and in particular a sensor of microelectromechanical system (MEMS) type.
Brief Discussion of the Related Art
An acceleration sensor having a vibrating resonator generally comprises a seismic body (or test mass) connected to a support by a vibrating element that is generally in the form of a beam extending along a sensing axis of the sensor. The sensor has transducers for setting the beam into vibration at the resonant frequency of the beam and for detecting variations in the frequency of vibration of the beam. Under the effect of an acceleration applied to the support, the seismic body exerts an axial force on the beam in compression or in traction: this leads to a change in the stiffness of the beam and thus to a change in its resonant frequency.
The acceleration measurement is thus deduced from a variation in the resonant frequency of the beam that is small, thereby leading to a measurement bias that is relatively large. Under the effects of acceleration, a force is applied to the resonator that changes its resonant frequency: the resulting deformation is relatively small and resembles interfering deformation as generated by variations in temperature or by stresses relaxing in the assemblies.
In order to eliminate this bias and other undesirable common mode effects (non-linearity, temperature sensitivity, . . . ), the measurements are performed differentially by replacing the single beam with two beams in a tuning fork configuration, which beams are subjected to stresses of opposite signs that are applied either by a seismic body that is common to both beams or else by two seismic bodies, each connected to a respective one of the beams.
In order to limit this measurement bias, proposals have also been made to implement electrostatic stiffness in parallel with the mechanical stiffness of the beam so that under the effect of an acceleration, the seismic body modifies the electrostatic stiffness.
In a sensor using this principle, the electrostatic stiffness is obtained by means of comb electrodes attached to each of the vibrating beams of a tuning fork in such a manner that under the effect of an acceleration the test body changes the airgap between the comb electrodes and thus changes the electrostatic stiffness generated by said electrodes. The change in the stiffness leads to a change in the frequencies of vibration of the beams. The beams have different frequencies of vibration and the difference between the two frequencies is a measure of the acceleration.
Increasing the amplitude of the variations thus makes it possible to reduce measurement bias.